As described in prior, commonly owned U.S. application Ser. Nos. 09/886,165, and 10/176,322, now issued as U.S. Pat. Nos. 7,104,747 and 7,204,923, respectively, and herein incorporated by reference, dielectrophoresis (hereinafter “DEP”) can be used to concentrate and filter particles suspended in a fluid. The dielectrophoretic force is produced by the action of an electric field gradient on a charge separation in particles suspended in an immersion liquid. This force is proportional to the real part of the relative difference in the complex conductivities of the particle and immersion liquid, and the square of the applied electric field. We have shown that insulators are practical and advantageous objects to produce the spatially non-uniform electric fields required for DEP.
More particularly, DEP is the motion of particles toward or away from regions of high electric field intensity. When an external electric field is applied to a system consisting of a particle suspended in a fluid medium, charges are induced to appear at the particle-fluid interface so as to confer on this polarized particle the properties of an electric dipole. The electrostatic potential of a polarizable particle is minimized in regions of highest electric field intensity. If the particles are immersed in a polarizable fluid, the electrostatic energy of the system is minimized by placing the most polarizable component in the high-field regions. If the particle is more polarizable than the fluid, it will be impelled toward a region of high field intensity (positive dielectrophoresis) or otherwise toward a region of lower field intensity (negative dielectrophoresis). The polarization of particles occurs by a variety of mechanisms having characteristic relaxation times. In DEP, the force on a particle and its surrounding medium is proportional to the gradient of the field intensity and is independent of the direction of the electric field. This is in contrast to electrophoresis, the field induced motion of charged particles, wherein the direction of the force on a particle is dependent upon the sign of the charge and the direction of the field.
We have also previously described a “faceted prism” method in commonly owned U.S. application Ser. No. 10/456,772, now issued as U.S. Pat. No. 7,005,301, entitled “Piecewise Uniform Conduction-like Flow Channels and Method Therefor”, and herein incorporated by reference. This “faceted prism” method describes a method for designing flow channels with uniform velocities throughout an electrokinetic flow field. The velocities remain uniform while turning and expanding channel flows to any value of turning angle and channel width. This is achieved by connecting deep and shallow sections of channels, wherein the channel depth varies abruptly along the interface between the adjoining sections in a ratio range of about 1:2 to about 1:1000. The method enables the selection of channel velocity in the shallow region relative to the velocity in the deep section. For ideal electrokinetic flows, the electrokinetic force on particles in the channel varies in direct proportion to the local channel velocity. Just as the velocity in each channel section is uniform, so is the electrokinetic force on a fluid particle uniform in each channel section. Therefore, by careful design of abrupt changes in specific permeability at an interface, the abrupt change in electrokinetic force can be selected. The desirable uniform velocity sections can also be designed to work with non-electrokinetic forces such as pressure-driven systems with Hele-Shaw designs. Moreover, combinations of fluid pumping methods such as electrokinetic and pressure-based devices can also be used to achieve the desired effect.
Because the abrupt interfaces also cause a sharp gradient in an applied electric field, a DEP force is established along the interface. Depending on the polarizability of the suspended particles, the DEP force can either complement or oppose the local electrokinetic force transporting the fluid through the channel. Moreover, for a transition in depth from deep to shallow channels, the DEP force will be the opposite of that for an abrupt transition in depth from shallow channels to deep channels.